By design, the inside of the spindle taper is harder than that of the toolholder’s shank. Therefore any “uniqueness” (not imperfections) in the spindle’s taper, no matter how slight, will be transferred to the toolholder. If you move that toolholder to another spindle, any of the new high spots on its taper will cold-weld to the new spindle surface. When the tool is removed that contact area will break off and appear as discoloration on the taper that is often mistaken for corrosion. That’s what we call fretting. It also leaves damage to the spindle taper that will transfer to the next toolholder that is loaded.
It’s a transmittable disease!
Dedicating new toolholders to a single spindle keeps the taper conformity contained and reduces fretting, extending the life of the holders and spindle.
Don't store toolholders like those in the photo above. Use stands, shelves or carts that keep the tapers separated and from being damaged. Wood shelving with holes cut in with a hole saw work great.
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